Consumers are increasingly demanding both faster and smaller electronic products. The use of PCBs has grown enormously as new electronic applications are marketed. A PCB is formed by laminating a plurality of conducting layers with one or more non-conducting layers. As the size of a PCB shrinks, the relative complexity of its electrical interconnections grows.
A via structure is traditionally used to allow signals to travel between layers of a PCB. The plated via structure is a plated hole within the PCB that acts as a medium for the transmission of an electrical signal. For example, an electrical signal may travel through a trace on one layer of the PCB, through the plated via structure's conductive material, and then into a second trace on a different layer of the PCB.
Some PCBs, such as backplanes and other thick-format printed circuit boards, can endure significant signal integrity (SI) disturbances or degradation as a result of unused portions of through-holes and vias that extend past their last connected layer. Known as “stubs,” these unused via portions result in signal reflections, capacitance, inductance and impedance discontinuities, and signal losses, all of which may have a negative impact on signal integrity as signal propagation speeds increase.
Such stubs may be avoided by forming blind vias or blind plated holes (BPH).
FIG. 1 illustrates a prior art approach for creating a blind plated hole. A sub-composite structure 109 comprises conductive layers 104a and 104b and dielectric layers 106a and 106b. A via hole 108 is drilled through the sub-composite structure 109, extending from a first surface 102 of the sub-composite structure 109 through a first conductive layer 112. The via hole 108 is plated with a conductive material 110, after which the via hole 108 is filled with a medium 118. After the via hole 108 is filled with the medium 118, one or more additional cores that make up the PCB, such as laminate 116, is added to sub-composite structure 109. Note that the medium 118 may serve to keep the hole 108 from otherwise filling with prepreg material when the laminate 116 is added to the sub-composite structure 109.
FIG. 2 illustrates the via hole 108 of FIG. 1 before and after backdrilling, and after the laminate 116 is added to the sub-composite structure 109. A part of the plated hole 108 (e.g., also referred to as a stub) is drilled out using a controlled depth drill with a larger drill bit diameter than the via hole 108. However, du has difficulty for the depth control when the PCB gets thicker format.
FIG. 3 illustrates a conventional high density interconnect PCB. The PCB 300 may be formed by separately forming two sub-assembly cores 302 and 304 and applying thicker than normal plating to through holes to form a plurality of vias 308 and 310. Those vias 308a, 308b, 308c, and 308d are back drilled to minimize stubs. The two sub-assembly cores 302 and 304 are then laminated together with enough prepreg 306 to fill via holes 308 and/or voids 312 left behind after drilling out the stubs. The external/exposed surfaces of the sub-assembly cores 302 and 304 may then be cleaned (scribed) to remove steeped resin. The sub-assembly cores 302 and 304 may then be drilled from their external/exposed surfaces to make plated blind holes.
However, this approach in FIG. 3 requires copper plating of the via holes that is sufficiently thick for subsequent drilling, which requires a longer plating time, limits the minimum dielectric/prepreg 306 thickness for the final assembly to deliver enough resin/prepreg to fill the holes 308a, 308b, 308c, and 308d, requires higher drill position accuracy, and potentially limits the achievable thickness range for the PCB 300 (e.g., it makes it more difficult to make thinner PCBs).
Therefore, a method is needed to efficiently form blind vias while reducing stub length and processing costs and difficulty.